<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <p>sub 表示下标</p>
    <p>H<sub>2</sub>O</p>

    <p>sup 表示上标</p>
    <p>E = mc<sup>2</sup></p>
</body>
</html>